Is this fair using Link exchange and private blog networks?

Currently, I am doing SEO for my client's website which is health niche. I have implemented all on-page factors. My keyword has very high competition and I was analyzing my competitors backlinks. Competitors who currently ranking well in Google.

Many of their backlinks come from private blog network and link exchange. I have read some post, blogs, articles on internet that these linking methods consider as unnatural link method from google.